An in vitro slice of the hippocampus from guinea pig brain was used to investigate the effect of repetitive stimulation of the perforant pathway on synaptic transmission from the perforant pathway to CA1 pyramical neurons. It was found that following stimulation of the perforant pathway at a frequency of 15 Hz for 15 sec., there was a long-lasting potentiation of synaptic excitation of CA1 neurons by perforant pathway stimulation.